This invention relates to light-emitting dendrimers and devices using them and methods for their synthesis and construction.
It has been clearly demonstrated that conjugated organic materials, including linear (co)polymers, oligomers, and molecular materials show considerable promise as the light-emitting layers in light-emitting diodes (LEDs) (J. H. Burroughs et al, Nature, 1990, 347, 539). However, to reach their full potential, a number of problems need to be overcome including achievement of the necessary emission colours (particularly blue and red), optimisation of chromophoric efficiency together with processing properties, and extension of device operational lifetime. The simplest organic-based LEDs have the organic light-emitting layer sandwiched between an anode, which injects holes, and a cathode, which injects electrons, with one or both of the electrodes being clear to allow the emission of light from the device. Most light-emitting conjugated materials tend to transport holes in preference to electrons and hence the LEDs which have been developed can incorporate additional charge transport layers in addition to the luminescent layer in an effort to balance the charge injection (A. R. Brown et al, Chem. Phys. Lett., 1992, 200, 46). The problem with using linear (co)polymers for such devices is that changes to the structure of the linear (co)polymer designed to change the electronic properties of the linear (co)polymer may then also change the synthetic procedure as well as the processing properties of the linear (co)polymers (P. L. Burn et al, J. Am. Chem. Soc., 1993, 115, 10117).
The present invention addresses the problems encountered with (co)polymers on the basis that dendritic light-emitting molecules will provide a new approach to the molecular engineering of materials for LEDs and tend to have distinct advantages over linear polymers in some or all of these areas, including efficiency, colour control, and processing.
Dendrimers are typically represented by a core (rectangle), dendritic branches including conjugated units (circles and triangles) and including branch links (L), and surface groups (S), of general structure shown in the accompanying FIG. 9. The branch links (L) can be a simple bond or bonds.
The dendrimers according to the present invention which can be used for incorporation into light emitting devices have a core and/or branches comprising electroluminescent or charge-transporting chromophores (conjugated units). The conjugated units used for the core and/or the branches need not be the same. The importance of each component of the dendrimer will be discussed below.
There have been many investigations into the synthetic procedures for the preparation of dendrimers. The dendrimers according to the invention may be synthesised by any convenient method, including both xe2x80x9cconvergentxe2x80x9d and xe2x80x9cdivergentxe2x80x9d methods (Z. Xu et al, J. Am. Chem. Soc., 1994, 116, 4537). In convergent methods the dendritic branches are first synthesised and then bonded to bonding sites on the core, whereas in divergent methods the dendritic branches are progressively built up from the core bonding sites.
Light-emitting diodes (LEDs) using as light-emitting element dendrimers having an anthracene core linked by acetylene (ethynyl) linkages to acetylene-linked 3,5-t-butylphenyl dendritic structures have been described by Jeffrey S. Moore et al in Adv. Mater. 1996, 8, No. 3, pp 237-241. Those dendrimers are believed not to be very efficient for the present purposes. Bettenhausen and Strohriegel, in Macromol. Rapid Commun. 17, 623-631 (1996) describe dendrimers having 1,3,4-oxadiazole linking units for use as electron injection and transport layers (not the light-emitting element) in light emitting devices.
S. K. Deb et al, in J.Am.Chem.Soc. (1997), 119(38), pp 9079-9080, have described reactive attachment of aldehyde-functionalised structures to the 1,3,5-positions of a benzene ring to synthesise poly(phenylenevinylene) dendrimers which are described as fluoroescent. These dendrimer end products, although not light emitting devices containing them, are excluded from the present invention, and so are the functionalised precursor structures.
The present invention provides dendrimers, ways of synthesising them, methods of processing them to make devices, and devices using them, which are believed to be superior to those previously known dendrimers, especially for use in light emitting devices. It is to be understood, for further avoidance of doubt, that the use of the specified dendrimers as a light-emitting element in a light-emitting device encompasses such use of the dendrimers either as the light-emitting element, or in the light-emitting element in the event that the light-emitting element is regarded as including structures or materials in addition to the dendrimers themselves.
One aspect of the invention accordingly provides a luminescent compound of the formula:
CORExe2x88x92[DENDRITE]n
in which CORE represents an atom or group, n represents an integer of at least 1 and DENDRITE, which may be the same or different if n is greater than 1, represents an at least partly conjugated dendritic molecular structure comprising groups selected from the group consisting of aryl and heteroaryl groups and alkenyl groups connected to each other via a carbon atom of an alkenyl group to a ring carbon atom of an aryl or heteroaryl group, CORE terminating in a first single bond which is connected to a ring carbon atom of an (hetero)aryl group to which more than one at least partly conjugated dendritic chain is attached, said ring carbon atom of an (hetero)aryl group forming part of DENDRITE. Preferably the compound emits light in the visible region under electrical or optical excitation. The compounds typically have one or more end or surface groups.
In any event, wherein (a) the CORE is not 
wherein X is bromine or CHO when n=2, (b) the CORE is not bromine or CHO when n=1 and is attached to 
and (c) the CORE does not contain the structure
xc3x8t-[xe2x80x94CHxe2x95x90CH-xc3x8p-CHxe2x95x90CHxe2x80x94]3
in which xc3x8t is a 1,3,5-bonded benzene ring and xc3x8p is a para-bonded benzene ring.